Generally, an inductor is needed in designing a radio-frequency integrated circuit (RF IC), for impedance matching. The quality factor and inductance of an inductor are important factors which determine the performance of a matching circuit. Inductors used for impedance matching, including off-chip inductors, bond wire inductors, spiral inductors, etc. are known in the related art. The off-chip inductor has the disadvantages of relatively large size and increased manufacturing costs. Although bond wire inductors exhibit excellent characteristics, large inductance deviation caused by problems in fabrication processes is a drawback. Therefore, spiral inductors, which exhibit a small inductance deviation, are most often used.
The spiral inductor includes a silicon substrate, a plurality of metal lines formed over the silicon substrate, an insulating layer for insulating the metal lines, and an inductor line. For an inductor to achieve a high quality factor, it is necessary to reduce parasitic resistance in the metal lines, and to reduce losses due to eddy currents and displacement currents in the substrate.
Generally, in the inductor, there is a capacitance between the area of the silicon substrate occupied by the metal lines and the grounded portion of the silicon substrate because the silicon substrate has a relatively-high dielectric constant. Due to the capacitance caused by the dielectric characteristics of the silicon substrate, energy is drained from the inductor through the grounded surface at the bottom of the silicon substrate.